Upon reaching Jill's location, a direct confrontation will ensue. This is a mini-boss fight against a human opponent. Jill is agile and uses several firearms. She will frequently dodge your attacks and use the environment for cover. Focus fire on her, and use grenades to flush her out. If you have a character with a stun ability (like Spectre's Threat Detector or Vector's Mimicry to get close), it can be very effective.
During the fight, waves of infected will also attack, creating a chaotic three-way battle. Prioritize Jill, but be mindful of the infected overwhelming your team. Once Jill's health is depleted, a cutscene will play, and she will be incapacitated. Your objective will then shift to securing her or eliminating her, depending on the choices made in the game (though for this walkthrough, we assume the standard path).